low temperature catalytic ethanol conversion over ceria-supported platinum, rhodium, and tin-based nanoparticle systems

Low Temperature Catalytic Ethanol Conversion

Over Ceria-Supported Platinum, Rhodium, and
Tin-Based Nanoparticle Systems



Thesis by

Eugene Leo Draine Mahmoud



In Partial Fulfillment of the Requirements for the Degree of
Engineer



California Institute of Technology
Pasadena, California



(Defended 2010)



1

2














3

Abstract
Due to the feasibility of ethanol production in the United States, ethanol has become more
attractive as a fuel source and a possible energy carrier within the hydrogen economy. Ethanol
can be stored easily in liquid form, and can be internally pre-formed prior to usage in low
temperature (200
o
C – 400
o
C) solid acid and polymer electrolyte membrane fuel cells. However,
complete electrochemical oxidation of ethanol remains a challenge. Prior research of ethanol
reforming at high temperatures (> 400

o
C) has identified several metallic and oxide-based
catalyst systems that improve ethanol conversion, hydrogen production, and catalyst stability.
In this study, ceria-supported platinum, rhodium, and tin-based nanoparticle catalyst systems
will be developed and analyzed in their performance as low-temperature ethanol reforming
catalysts for fuel cell applications.
Metallic nanoparticle alloys were synthesized with ceria supports to produce the catalyst
systems studied. Gas phase byproducts of catalytic ethanol reforming were analyzed for
temperature-dependent trends and chemical reaction kinetic parameters. Results of catalytic
data indicate that catalyst composition plays a significant role in low-temperature ethanol
conversion. Analysis of byproduct yields demonstrate how ethanol steam reforming over
bimetallic catalyst systems (platinum-tin and rhodium-tin) results in higher hydrogen selectivity
than was yielded over single-metal catalysts. Additionally, oxidative steam reforming results
reveal a correlation between catalyst composition, byproduct yield, and ethanol conversion. By
analyzing the role of temperature and reactant composition on byproduct yields from ethanol
reforming, this study also proposes how these parameters may contribute to optimal catalytic
ethanol reforming.

4

Table of Contents
1 Introduction and Theory 5
1.1 Thermochemistry of Ethanol Reforming 6
1.1.1 Steam Reforming 6
1.1.2 Partial Oxidation 7
1.1.3 Oxidative Steam Reforming 8
1.1.4 Additional Ethanol Reforming Reactions 8
1.2 Metal Catalysts for Ethanol Reforming 9
1.3 Oxides as Catalysts and Metal Catalyst Supports 11
1.4 Multi-Component Catalyst Systems 12

1.5 Proposed Work 14
2 Experimental Approach 16
3 Results and Data Analysis 19
3.1 Steam Reforming 22
3.1.1 Rh
x
Sn
1-x
/CeO
2
, (x = 1, 0.9, 0.8) 22
3.1.2 Pt
x
Sn
1-x
/CeO
2
, (x = 1, 0.9, 0.8) 26
3.1.3 Activation Energies for Rate-Determining Reactions 30
3.2 Oxidative Steam Reforming 32
3.2.1 Rh
x
Sn
1-x
/CeO
2
, (x = 1, 0.9, 0.8) 32
3.2.2 Pt
x
Sn

1-x
/CeO
2
, (x = 1, 0.9, 0.8) 36
3.3 Ethanol Reforming with Varying Reactant Composition 39
4 Conclusion and Future Work 45
5 Acknowledgements 48
6 References 50
7 Appendix: Plots of Ethanol Reforming Byproducts for Catalysts Studied 55
7.1 Ethanol Reforming over Rh (5% wt.)/CeO
2
56
7.2 Ethanol Reforming over Rh
9
Sn
1
(5% wt.)/CeO
2
60
7.3 Ethanol Reforming over Rh
8
Sn
2
(5% wt.)/CeO
2
65
7.4 Ethanol Reforming over Pt (5% wt.)/CeO
2
68
7.5 Ethanol Reforming over Pt

9
Sn
1
(5% wt.)/CeO
2
72
7.6 Ethanol Reforming over Pt
8
Sn
2
(5% wt.)/CeO
2
78


5

1 Introduction and Theory
Steam reforming is a thermochemical process in which large hydrocarbon molecules are broken
down into hydrogen gas (H
2
), smaller oxides, and hydrocarbons. Steam reforming of natural
resources is the primary process for the industrial production of hydrogen gas in the world.
About 50% of the world’s production of hydrogen gas and 95% of hydrogen gas production in
the United States is generated from steam reforming of natural gas [13]. When synthesized at a
large scale, steam reforming typically employs a catalyst and high temperatures (> 600
o
C), and is
the most energy efficient and cost efficient means of producing hydrogen gas.
Steam reforming of alcohols has been proposed as a primary means of hydrogen production for

fuel cell devices. Fuel cells are advantageous as energy conversion devices for several reasons.
They are more energy efficient than Carnot-limited combustion engines. When using hydrogen
gas as a fuel source, the only byproduct produced is water vapor (H
2
O). Also, the performance
of low temperature (< 100
o
C) proton exchange membrane fuel cells is suitable for a wide range
of mobile applications. Identifying an appropriate source for hydrogen production will solidify
the role of fuel cells in the energy marketplace.
As a means to address concerns over energy security, sustainability of energy sources, and
global climate change, using a non-petroleum-based energy carrier for fuel cells is critical [11].
Ethanol (CH
3
CH
2
OH) is attractive as a feedstock for hydrogen gas production, in part, because of
its ample production domestically—composing 99% of biofuel production in the United States.
Also, ethanol can be produced renewably, it is low in toxicity, it can be easily transported, and it
has a relatively high energy density. Thus the catalytic reforming of ethanol provides a plausible
means of hydrogen gas production for the forthcoming fuel cell industry. For certain


6

intermediate temperature (200
o
C–400
o
C) fuel cells, internal reforming of ethanol could improve

reforming efficiency while removing the challenges of hydrogen gas storage from the fuel cell
system. The objective of this section of the thesis is to delineate the different approaches and
reactions incorporated in ethanol reforming, to discuss the advantages of oxide-supported
metal catalysts for hydrogen gas production, and to discuss how multi-component catalysts may
offer improvements in catalytic ethanol reforming.
1.1 Thermochemistry of Ethanol Reforming
The main approaches to ethanol reforming for fuel cells are external reforming, integrated
reforming, and internal reforming [23]. In external reforming, the conversion to hydrogen takes
place in a separate reactor, and the resultant fuel is fed into the fuel channels. These catalytic
systems may be able to benefit from the fuel cell stack’s waste heat, but in general, they
operate as technologically mature independent systems. Integrated reforming involves some
arrangement in which the membrane electrode assembly (MEA) and the reformer are
alternatively arranged within the fuel cell stack. This approach benefits from a close thermal
contact between MEA and the reformer. Internal reforming requires the direct incorporation of
a reformate layer into either the fuel channel and/or anode. This approach ensures maximum
thermal efficiency and a coupling of all reforming byproducts into the anode’s electrochemical
reactions.
Several reaction pathways are available to ethanol reforming, and the thermodynamics of these
reactions are presented in the remainder of this section.
1.1.1 Steam Reforming
The most desirable form of the steam reforming (SR) reaction is endothermic and produces only
hydrogen and carbon dioxide (CO
2
).

7

CH
3
CH

2
OH(l) + 3 H
2
O(l)  6H
2
(g) + 2CO
2
(g) ∆H
298
= +347.5 kJ mol
-1
(1)

The two other steam reforming reactions produce less desirable byproducts—carbon monoxide
(CO) and methane (CH
4
)—in exchange with hydrogen or carbon dioxide [3].
CH
3
CH
2
OH(l) + H
2
O(l)  4 H
2
(g) + 2CO(g) ∆H
298
= +341.7 kJ mol
-1
(2)

CH
3
CH
2
OH(l) + 2 H
2
(g)  2 CH
4
(g) + H
2
O(g) ∆H
298
= −114.0 kJ mol
-1
(3)

Given that reactions (1) and (2) are endothermic and increase the amount of moles in the
system, SR conditions at high temperatures (> 700
o
C) will favor hydrogen production and the
methane producing reaction (3) will be less favorable. In the comparison of reactions (1) and
(2), the higher (3:1) molar ratio of water-to-ethanol in reaction (1) favors the production of CO
2

as opposed to CO.
1.1.2 Partial Oxidation
When a sub-stoichiometric amount of oxygen gas (O
2
) is present in the reactant mixture with
ethanol, an exothermic reaction produces carbon dioxide and hydrogen.

CH
3
CH
2
OH(l) + 1.5 O
2
(g)  3 H
2
(g) + 2CO
2
(g) ∆H
298
= -510.0 kJ mol
-1
(4)

Less than ideal reactions that may occur during partial oxidation (PO) conditions would result in
the production of carbon monoxide and/or water vapor [28].
CH
3
CH
2
OH(l) + 0.5 O
2
(g)  3 H
2
(g) + 2CO(g) ∆H
298
= +55.9 kJ mol
-1

(5)
CH
3
CH
2
OH(l) + 2 O
2
(g)  3 H
2
O(g) + 2CO(g) ∆H
298
= -669.6 kJ mol
-1
(6)


8

PO allows for ethanol reforming at lower temperatures (i.e. without heat input) and without the
presence of steam. However, reaction (4) inherently exhibits a lower hydrogen selectivity—
moles of hydrogen produced per mole of ethanol consumed—then is does reaction (1).
1.1.3 Oxidative Steam Reforming
Oxidative steam reforming (OSR) occurs when steam reforming and the partial oxidation
reaction conditions are coupled. The OSR reaction, also known as autothermal reforming
reaction, results in the production of hydrogen and carbon dioxide with only a small change in
the system’s enthalpy.
CH
3
CH
2

OH(l) + 1.8 H
2
O(l) + 0.6 O
2
(g)  4.8 H
2
(g) + 2CO
2
(g) ∆H
298
= +4.5 kJ mol
-1
(7)

The hydrogen selectivity for reaction (7) is slightly lower than that of reaction (1). However, the
slight change in the system’s enthalpy would make this equation more sustainable at low
temperatures.
1.1.4 Additional Ethanol Reforming Reactions
Besides the primary reactions described above, other likely reactions include ethanol
decomposition, water gas shift (WGS), ethanol dehydrogenation, ethanol dehydration, and
methanation reactions [33].
CH
3
CH
2
OH(l)  CO + CH
4
+ H
2
ΔH

298
= +91.8 kJ mol
-1
(8)
CH
3
CH
2
OH(l)  0.5 CO
2
+ 1.5 CH
4
ΔH
298
= -31.7 kJ mol
-1
(9)
CO + H
2
O  CO
2
+ H
2
ΔH
298
= -41.1 kJ mol
-1
(10)
CH
3

CH
2
OH(l)  H
2
+ CH
3
CHO(l) ΔH
298
= +84.8 kJ mol
-1
(11)
CH
3
CH
2
OH(l)  C
2
H
4
(g) +H
2
O(g) ΔH
298
= +87.6 kJ mol
-1
(12)
CO + 3H
2
 CH
4

+ H
2
O ΔH
298
= -206 kJ mol
-1
(13)

9

Due to the many possible reaction pathways that are available for ethanol steam reforming, it is
important to identify which reactions are the most likely to occur and to catalytically promote
the reactions that most strongly favor the production of hydrogen and carbon dioxide. Reaction
(9) is strongly favored at low temperatures (~ 200
o
C), and may dominate over reactions (1) and
(2). The water-gas-shift reaction (10) strongly favors the conversion of carbon monoxide to
carbon dioxide in the presence of steam [11]. This reaction is an important step in purifying
steam reforming byproducts, particularly because the production of carbon monoxide can result
in the poisoning or deactivation of certain metal catalysts typically used in reformers and fuel
cell anodes. Carbon formation is also a reaction that may result from the presence of carbon-
containing byproducts. Coking may result from the Boudouard reaction, the decomposition of
methane and hydrocarbon polymerization.
2CO  C(s) + CO
2
ΔH
298
= -172 kJ mol
-1
(14)

CH
4
 2H
2
+ C(s) ΔH
298
= +74.6 kJ mol
-1
(15)
C2H4  polymers  coke (16)

Designing a catalyst system in which these coking reactions are limited is crucial for the
development of a stable and active ethanol reforming catalyst. The challenge has led to
increasing research in the development of stable and active catalysts for ethanol reforming.
1.2 Metal Catalysts for Ethanol Reforming
Typically, ethanol reforming is carried out at high temperatures (> 600
o
C). An ideal catalyst
system for low-temperature ethanol reforming would be stable, highly selective to H
2
, and
composed of accessible materials. Noble metal catalysts have typically been used in industrial
catalytic reformers to produce hydrogen from ethanol.

10

Platinum-based catalysts are well-known for being active in the electrochemical oxidation of
alcohols. Ethanol reforming to hydrogen over platinum (Pt) is promoted via ethanol
decomposition (8) and ethanol dehydration (11, 12) [7, 8, 15, 20, 21, 39]. However, ethanol
reforming at lower temperatures (< 500

o
C) generally leads to catalyst deactivation with
acetaldehyde and methane as the primary byproducts. In particular, the low selectivity to
hydrogen in favor of carbon monoxide suggests that the platinum surface promotes the reverse
water gas shift reaction. Palladium-based catalysts tend to promote similar reaction
byproducts, although activity at low temperatures is lower than platinum [14].
Rhodium (Rh) has been shown to be the most active, stable, and resistant to sintering amongst
oxide-supported noble metals catalysts for ethanol reforming [12]. Rhodium is an efficient
metal catalyst that is active in breaking the carbon–carbon (C-C) and hydrocarbon (H-C) bonds of
possible intermediates—such as acetaldehyde and oxametallacycles—during ethanol steam
reforming [22]. As shown in Figure 1, an oxametallacycle refers to the five-member adsorbed
complex formed by the insertion of a metal dimer into one of the C-O bonds of an ethylene
oxide molecule (C
2
H
4
O). The stability of these structures favors the breaking of the C-C bond,
particularly in an oxidizing atmosphere [28]. Given that oxametallacycles are more energetically
favorable on the surface of rhodium than adsorbed acetaldehyde, ethanol decomposition on
rhodium is more likely to promote C-C bond rupture. However, hydrogen selectivity over
rhodium catalysts varies with fabrication, loading, and oxide support. Oxide support is
particularly critical at low temperatures (< 500
o
C) [10], at which methane and carbon monoxide
production are significant amongst reforming products. This suggests that Rh alone is not
catalytically active enough to efficiently produce hydrogen at low temperatures.

11

Figure 1: Formation of an adsorbed oxametallacycle from an adsorbed ethylene oxide molecule
Amongst non-noble metals, nickel catalysts have also been used in ethanol reforming because
of their known activity in oxidation reactions, their low-temperature activity in dehydrogenation
reactions, and their low cost [30, 31]. Cobalt catalysts have demonstrated peak hydrogen
selectivity at 450
o
C with a CeZrO
4
support [19] and are active in the breaking of the carbon-
carbon bond. However, particle size and coking are factors that limit the stability for both of
these metal catalysts at low temperatures.
1.3 Oxides as Catalysts and Metal Catalyst Supports
Despite the prevalent role of metals in catalytic reactions, studies of oxide materials have
demonstrated their ability to act as catalysts and to enhance the performance and stability of
metal catalysts [3]. The choice of a support material can favor other secondary reactions—such
as water splitting into hydroxyl (OH) groups and hydrogen radicals—and can promote the
migration of these reactive species toward the metal particles. Support materials can also aid in
the dispersion and thermal stability of metal particles.
Cerium oxide or ceria (CeO
2
) has garnered interest in the material science community for its
ability to participate in homogeneous catalytic reactions [1, 13, 27, 32], such as three-way
catalysis (TWC) and fluid catalytic cracking. The stoichiometric form of ceria is a face-centered
cubic cell with a fluorite structure. When treated in a reducing atmosphere at elevated

12

temperatures, a continuum of oxygen-deficient non-stoichiometric oxides are formed. These
suboxides are readily reoxidized to CeO
2

in an oxidizing environment. The ability of CeO
2
to
release and store oxygen allows for improved performance from nearby catalysts—such as in
the water-gas shift reaction (10) and ethanol dehydrogenation (11). As a metal support, oxides
and metals have a synergistic relationship. Precious metals promote the reduction and
oxidation of CeO
2
, while CeO
2
stabilizes the dispersion of the precious metal and resists
sintering. Other commonly used supports in steam reforming reactions include aluminum oxide
(Al
2
O
3
) [2, 40], magnesium oxide (MgO), titanium oxide (TiO
2
) [24,29], zinc oxide (ZnO) [4], and
zirconium oxide (ZrO
2
) [5, 19].
1.4 Multi-Component Catalyst Systems
In an effort to enhance catalytic activity, catalyst development has been increasingly employing
smaller catalyst particle size and metal alloys instead of single metal catalysts. The bifunctional
theory of electrocatalysis was proposed by Watanabe and colleagues [35—37] to account for
the change in electrocatalytic activity of these multi-component systems. This theory is
presupposed on the mixture of electrocatalysts—with different adsorption properties—on the
atomic scale. Watanabe’s work demonstrated how oxidation of organic molecules over
platinum was improved by the atomic level addition of other electrocatalysts (i.e., gold,

ruthenium) that could access lower energy pathways for the adsorption of reactive species.
Effectively, one metal acts as sites for organic species and another metal acts as sites for
oxygen-containing species. Complex reactions involving various species and reaction pathways
will thus occur more efficiently at metal interfaces.
Given the unique performance of multi-phase nanoparticles catalyst systems, there has been an
increasing effort by researchers to identify and describe the varied and synergistic roles of metal

13

and oxide catalyst materials. DeSouza and colleagues conducted a study of ethanol oxidation
over a PtRh
x
alloy electrode [9]. Using differential electrochemical mass spectroscopy (DEMS)
and Fourier transform infrared spectroscopy (FTIR), their work demonstrated that the addition
of Rh to a Pt catalyst increases the selectivity towards the complete oxidation of ethanol to CO
2
,
while decreasing selectivity to acetaldehyde. Ethanol oxidation requires C-H bond and C-C bond
dissociation, in addition to CO-O bond coupling. DeSouza’s work suggests that because Pt has a
relatively low bond energy for CO and O adsorption, Pt and PtRh
x
catalysts are more likely than
Rh to have a lower CO
2
activation energy. A linear sweep voltammetric study of adsorbed CO
suggests that Rh ad-atoms modify the electrocatalytic properties of Pt to promote the partial
oxidation of CO [8]. While in a bimetallic system, Rh continues to play the role one would
expect it to perform in a single catalyst system. A mechanistic study of PtRh
x
confirms that while

Rh allows for the formation of adsorbed oxametallacycles, and thus carbon-carbon bond
decomposition, an additional metal (Pt, Pd) is necessary for efficient hydrogen production [28].
In addition, the presence of a CeO
2
support also favors the dehydrogenation of ethanol to
acetaldehyde.
Platinum-tin alloys also participate in the oxidation of ethanol and catalytic promotion of CO

partial oxidation. Dissociative adsorption of water molecules on tin (Sn) allows for OH species to
interact in the dissociative adsorption of ethanol, into CO
2
and CH
3
COOH [34]. Additionally,
numerical calculations suggest that the CO oxidation potential on PtSn
x
is lower then the
oxidation potential on Pt. However, this has yet to be experimentally confirmed. An
electrochemical characterization of PtSn
x
and PtSn
x
O
y
electrodes by Jiang and colleagues [16]
shows that ethanol oxidation and hydrogen selectivity is more favorable on PtSn
x
O
y
. One

possible explanation is that tin oxide particles near Pt particles act as oxygen donor sites for the
CO-O bond coupling.

14

Recent studies of ternary catalyst systems continue to offer new insights into the roles that Pt,
Rh, and Sn play in ethanol reforming. A study of ethanol oxidation over a carbon-supported
Pt
6
Sn
3
Ru
1
showed high performance relative to the ethanol oxidizing ability of other binary and
ternary catalyst systems considered [38]. The presence of the PtSn phase and the SnO
2
were
identified as active structures in C-C bond dissociation. Ribeiro and colleagues considered the
addition of iridium [25] and tungsten [26] to a carbon-supported PtSn binary system. Both
materials enhanced the electrocatalytic activity of PtSn, possibly through some synergistic
structural arrangement with Sn, or by limiting ethanol adsorption in favor of oxygen containing
species. Several studies of PtRh
x
Sn
y
electrodes system have touted their performance as
ethanol oxidation catalysts [6, 17]. However, further studies of PtRh
x
, PtSn
x

, and PtRh
x
Sn
y

catalysts as low-temperature ethanol reformers will be needed to identify the optimal material
for low-temperature reforming and fuel cell conditions.
1.5 Proposed Work
The ideal ethanol reforming catalyst will be highly selective to hydrogen, with a low selectivity to
methane, acetaldehyde, and a minimal production of CO and other large hydrocarbon
complexes while operating in reactor at low temperatures (200
o
C–400
o
C) and atmospheric
pressures. Designing an optimal catalyst for hydrogen production from ethanol requires
consideration of the catalyst fabrication technique, proper choice of catalyst components,
support structure, and careful definition of the reforming environment. Catalyst design is
particularly critical in the low temperature regime, where reaction kinetics often plays a role
larger than thermodynamics. In this study, ceria-supported, 5%-weight Pt, Rh, PtSn
x
, and RhSn
x
catalysts will be fabricated and analyzed in their performance as low-temperature ethanol
reforming catalysts for fuel cell applications. We will discuss trends in ethanol reforming over
these catalyst systems, identify reaction kinetic parameters for the production of the ethanol

15

reforming byproducts that we detect, and propose future studies to help identify an optimal

ethanol reforming catalyst.


16

2 Experimental Approach
The catalyst materials used were composed of Pt
x
Sn
1-x
or Rh
x
Sn
1-x
nanoparticles with x = 1, 0.9
and 0.8 on a porous ceria support (95% by weight). The catalysts were fabricated at Occidental
College by Marc Sells, under the supervision of Dr. Adrian Hightower. The mass of each catalyst
sample studied is shown in Table 1. A modified version of reverse micelles synthesis was used
to produce platinum, rhodium, and tin nanoparticles with diameters ranging from 1–10 nm.
Nanoparticles were dispersed on the surface ceria supports, and the resulting cermet powder
was washed, dried, mixed with quartz sand (10 parts by volume), and mounted into a 0.8 inch
diameter plug flow reactor—as shown by the diagram in Figure 2. The catalyst material was
sandwiched in between two fine porous quartz cylinders—one fused to the end of the reactor
tube and the other slip fit into the inlet side of the tube. This setup ensures that inlet gases pass
through the catalyst system at a known rate. A thermocouple was mounted through the inlet-
side quartz cylinder to monitor the catalyst sample temperature, and a horizontal Carbolite tube
furnace was used to heat the reactor.
Table 1: Masses of catalyst material used in ethanol reforming studies
Rh Rh
0.9

Sn
0.1
Rh
0.8
Sn
0.2
Pt Pt
0.9
Sn
0.1
Pt
0.8
Sn
0.2
Catalyst
mass used
217 mg 235.4mg 223.5 mg 220.4 mg 254mg 263.5 mg


17


Figure 2: A schematic of the tube furnace reactor setup
Reactant gases and reactant products were ported through the valves at the reactor’s end caps.
The water-to-ethanol gas phase molar ratio of the reactant was set to a desired value by
premixing the liquids in a bubbler, and heating the mixture to a predetermined temperature (≈
70
o
C). A rotameter was used to flow argon through the bubbler as a means of transporting a
vapor mixture of steam and ethanol to the catalyst. Additional rotameters were used to

transport nitrogen, oxygen, and additional argon to the catalyst bed within the tube furnace
reactor. The measurement of the volume of reactants used and the accuracy of the rotameters
were the primary sources of systematic error observed with results of this study. A liquid
vaporizer was considered for the transport of ethanol, but this method could not yield the large
reactant flow rates required. Flow meters typically operate in much larger ranges than we
required, and flow rates produced were less accurate.
After installing the catalyst in the reactor tube, the catalyst bed was preheated to 400
o
C at 5
o
C
per minute under flowing argon for two hours, and then reduced under a 2% H
2
flow at a rate of
120 mL/min for 10 hours at the same temperature. Prior to testing the catalyst, the reactor
tube outside of the tube furnace was heated to 200
o
C by heating tape. The inlet line from the

18

bubbler, the outlet line to the GC, and the reactor end caps were heated to a temperature
between 70
o
C and 100
o
C. The reducing flow was removed from the reactor at least an hour
prior to the catalytic studies and gas chromatography was used to confirm that argon was the
only gas present. Catalytic studies were allowed 30 minutes to reach equilibrium prior to the
initial recording of data. Results were averaged over a 30 minute period.

A liquid trap was maintained at a set temperature (i.e. 30
o
C) and was installed at the reactor
outlet and was used to condense saturated ethanol vapor, saturated water vapor, and any other
saturated vapor byproducts. The remaining gas phase products entered the Varian CP-4900 gas
chromatograph with Molecular Sieve 5A and Porapak Q columns running on argon carrier gas.
Product gas compositions (H
2
, CO, CO
2
, CH
4
, C
2
H
4
, C
2
H
6
, O
2
, and CH
3
CHO) were obtained directly.
Ethanol vapor was also detected by the gas chromatograph. This ethanol vapor is the amount of
unsaturated ethanol vapor at the outlet of the reactor, and thus was not condensed in the liquid
trap. Thus, the product gas composition of ethanol was determined by correlating the amount
of ethanol gas vapor detected to the known vapor pressure of ethanol at the liquid trap’s
temperature. Alternatively, an absorption tube was installed at the reactor outlet to capture all

catalytic products. The tube was then purged with helium gas and analyzed for gas and liquid
phase products using the GC/MS setup (Hewlett-Packard 6890 GC -5973 MSD System) in
Caltech’s Environmental Analysis Center under the supervision of Dr. Nathan Dalleska. Kinetic
reaction parameters reported were obtained by numerical fitting of experimental data in the
Origin 6.1 software package.



19

3 Results and Data Analysis
Steam reforming (SR) of ethanol was studied using a reactant mixture with a molar water-to-
ethanol ratio of 3:1, which corresponded to the ideal stoichiometric ratio for the production of
hydrogen (1). The ratio of catalyst weight to reactant flow rate was 7.8 (kg / m/sec)
.
Ethanol
vapor, steam, and argon flow rates were set at 10, 31, and 120 sccm, respectively. These
conditions produced a Gas Space Hourly Velocity (GHSV)—defined as the milliliters of reactant
flow per hour per milliliters of catalyst used—of approximately 5000 hr
-1
at 400
o
C over ceria-
supported Pt. The total amount of ethanol converted was the difference between the ethanol
flow rate at the inlet and the detected ethanol flow rate at the outlet. While hydrogen
selectivity would be maximized at lower reactant feed rates (and lower GSHV), low reactant
conversion (≈ 25%) and a smaller slope in molar-selectivity-to-GSHV is required to accurately
study kinetic reaction data in the differential regime of the reactor. Figure 3 shows a plot of
GSHV versus ethanol conversion and hydrogen selectivity using steam reforming conditions over
a ceria-supported platinum catalyst. At a GSHV of 5000 hr

-1
the steam reforming reaction
should occur in the differential regime of the reactor. Systemic error for steam reforming
measurements is represented by error bars of 5.4%. Product gases will be presented in units of
molar selectivity, defined in this study as the ratio of moles produced for a certain byproduct to
the moles of ethanol converted.



20


Figure 3: Plot of hydrogen selectivity and ethanol conversion versus reactant residence time over a Pt/CeO
2

catalyst. Temperature is 400
o
C, and the reactant mixture has a water-to-ethanol ratio of 3:1.

Oxidative steam reforming (OSR) of ethanol was studied using a water-to-ethanol-to-oxygen
molar ratio of 1.8:1.0:0.6, which corresponds to the optimal stoichiometric ratio for the
production of hydrogen (7). The ratio of catalyst mass to reactant flow rate was 6.1 (kg /
m/sec). Ethanol vapor, steam, oxygen, and argon flow rates were set to 15.7, 28.2, 9.4, and 120
sccm, respectively. These settings produced a GSHV of approximately 6400 hr
-1
at 400
o
C over
ceria-supported Pt. Ethanol conversion at these conditions is around 40% (see Figure 4), but
hydrogen selectivity is relatively low and appears independent of residence time at these

conditions. Thus, OSR at a GSHV of 6400 hr
-1
should fall within the differential regime of the
reactor and allow for accurate calculation of kinetic information. Systemic error for oxidative
steam reforming measurements is represented by error bars of 7.1%.

21


Figure 4: Plot of hydrogen selectivity and ethanol conversion versus reactant residence time over a Pt/CeO
2

catalyst. Temperature is 400
o
C, and the reactant mixture has a water-to-ethanol-to-oxygen molar ratio of
1.8:1.0:0.6.

Reforming byproducts for steam reforming and oxidative steam reforming were studied at 50
o
C
intervals between 200
o
C and 400
o
C over ceria-supported platinum, rhodium, platinum-tin, and
rhodium-tin catalysts using the flow rates information given above. For results using different
reactant flow conditions, reactant composition will be noted in subsequent sections. Reforming
byproducts were analyzed for trends resulting from the varying the stoichiometric reactant
compositions. The steam, ethanol or oxygen concentration in the reactant mixture was varied,
while the remaining reactant components were held constant. During these studies,

temperature was held constant at 400
o
C.


22

3.1 Steam Reforming
3.1.1 Rh
x
Sn
1-x
/CeO
2
, (x = 1, 0.9, 0.8)
Selected results of the ethanol steam reforming studies over Rh-based catalysts are presented in
Figures 5–10. Ethanol conversion is one of the primary means for comparing the performance
of a catalyst, as it correlates the efficiency of the complex reaction mechanisms that produce
the byproducts detected. The conversion of ethanol varied between 20% and 40%, generally
increasing with increasing temperature over Rh, while slightly decreasing with increasing
temperature over Rh
8
Sn
2
. Ethanol reforming over Rh
9
Sn
1
produced the highest ethanol
conversion (30–35%) amongst Rh-based catalysts. Selectivity for hydrocarbons and carbon

dioxide remained minimal (< 0.1) for all Rh-based catalyst systems and temperatures, although
selectivity for these products was enhanced by increasing temperature. As shown in Figure 8,
selectivity to smaller hydrocarbons (methane, ethylene) and carbon dioxide was higher with
steam reforming over the Rh catalyst, while selectivity to acetaldehyde (ethanal/CH
3
CHO) was
higher with steam reforming over Rh-Sn catalyst systems. The primary products observed in
these steam reforming studies were hydrogen and carbon monoxide. Carbon monoxide
selectivity increased with increasing temperature across all catalyst systems studies (0.1–0.2).
Hydrogen selectivity increased with temperature for the Rh and the Rh
8
Sn
2
catalyst. Selectivity
increased more slowly and exponentially for the Rh
9
Sn
1
catalyst. At 400
o
C, production and
selectivity to hydrogen was highest for Rh
8
Sn
2
, followed by Rh
9
Sn
1
and Rh. The Rh

9
Sn
1
catalyst
had the lowest hydrogen selectivity in the group at temperatures below 350
o
C, while the Rh
8
Sn
2

catalyst had the highest selectivity above 300
o
C. Finally, by calculating the difference between
the moles of carbon in the reactants and the moles of carbon in the products, the moles of
undetected carbon-containing products can be estimated. This difference can be attributed to
the selectivity for carbon-containing products that were not detected by our experiment (i.e.,

23

benzene, etc.), and this selectivity has been represented with other reforming products in the
figures below. Selectivity to undetected carbon-containing products for all the Rh-based
catalyst systems decreased with increasing temperatures, and this decrease likely corresponds
strongly to the production of other byproducts.



Figure 5: Plot of molar selectivity and ethanol conversion versus temperature over Rh/CeO
2
from steam reforming

24


Figure 6: Plot of molar selectivity and ethanol conversion versus temperature over Rh
9
Sn
1
/CeO
2
from steam
reforming

Figure 7: Plot of molar selectivity and ethanol conversion versus temperature over Rh
8
Sn
2
/CeO
2
from steam
reforming